1. Field of the Invention
The present invention relates to a noise masking system and method in an image forming apparatus such as, for example, an office automation (OA) apparatus, a laser beam printer, or an electrophotographic copying machine, using drive motors as drive sources for operation. The noise masking system and method according to the present invention generates a masking sound for cancelling noises generated from the drive motors which noises cause an unpleasant feeling.
2. Description of Related Art
In a conventional image forming apparatus such as a laser beam or an electrophotographic copying machine there are used a plurality of mechanical drive motors, which are special drive motors developed along the recent tendency to digitization.
For example, in a digitized image forming apparatus, the reading of an image is performed by scanning an image carrier (original) with a light source, e.g. light emitting diode (LED), and reading the image by a charge coupled device (CCD). For recording an image there is used an image recorder, which scans a recording medium with light beam emitted from a light source, e.g. laser diode, and modulated by an image signal or a character signal and then records the image (prepares an original). In this case, as an optical scanner for the light beam there is used an optical deflector. The optical reflector comprises a rotary polyhedron mirror having a plurality of reflective surfaces on the outer periphery thereof and a drive motor for rotating the said rotary polyhedron mirror. An example of the drive motor used in such an optical deflector will be described below.
FIG. 26 is a perspective view explaining the construction of an optical deflector (optical scanner). In the same figure, numeral 51 denotes a drive motor, numeral 52 denotes a rotary polyhedron mirror, 53 a laser beam source, 54 a collimator lens, 55 a light condensing optical component (condenser lens), and 56 a recording member (photosensitive drum).
The construction of such an optical deflector used in an image forming apparatus, as well as an image recording method, will now be described with reference to FIG. 26. In recording an image, the rotary polyhedron mirror 52 is rotated in the direction of arrow A by the drive motor 51. The laser beam source 53 is constituted by a laser such as a semiconductor laser or a gas laser. Light beam emitted from the laser light source 53 is modulated with an image signal by means of a modulator (not shown) and the thus-modulated light beam is incident on one reflecting mirror surface of the rotary polyhedron mirror 52 through the collimator lens 54. The light beam reflected by the reflecting mirror surface of the rotary polyhedron mirror 52 is projected on the recording member 56 through the light condensing optical component 55. In this case, with rotation of the rotary polyhedron mirror 52 in the direction of arrow A, the reflected light beam is deflected in the direction of arrow B and scans the recording member 56 horizontally. Along with this horizontal scanning, the recording member 56 is rotated in the direction of arrow C, whereby a vertical scanning is performed. In this way a two-dimensional image is written onto the recording member 56.
The drive motor 51 used in such an optical deflector is of the type in which there is used a rotary bearing such as a dynamic pressure air bearing or ball bearing, using one of a sleeve and a shaft both fitted together as a rotating member and the other as a stationary member, and a rotational torque is generated by a magnetic circuit composed of a permanent magnet attached to the rotating member and an electromagnetic coil wound round an annular iron core mounted in the stationary member. Thus, the drive motor 51 has a magnetic circuit functioning also as a magnetic bearing which holds a rotor in the axial direction.
Consequently, when the image recording described above is performed, there arises a noise upon operation of the drive motor 51. A description will now be given of noises which occur with change in the number of revolutions of the drive motor. As shown in FIG. 27, a noise occurs and changes as the number of revolutions of the drive motor in the optical scanner changes. The timing chart of FIG. 27 shows noises occurring in the process from when the power is turned ON until when a series of image forming operations are completed. This change in the noise level is almost the same as the change in the number of revolutions of the drive motor acting as a main component of the drive mechanism. It is FIG. 28 that explains the change in the number of revolutions of the drive motor 51 alone. In FIG. 28, it is not the noise level (dB) but the number of revolutions, f, that is plotted along the axis of ordinate. The value of dB (loudness) itself changes little even with a change in the number of revolutions. A change in the frequency (timbre) of noise which occurs with a change in the number of revolutions is offensive to the ear.
As shown in FIG. 28, upon turning ON of the power, the number of revolutions of the drive motor is increased up to a predetermined value. If a predetermined processing is not started after continuance of the predetermined number of revolutions, a stand-by mode starts, in which the number of revolutions is decreased and the motor assumes a rest state. Thereafter, when the start of the processing is instructed, the drive motor starts to rise, and when the number of revolutions of the drive motor has reached a predetermined value, the drive motor starts to operate for the predetermined processing. Then, upon termination of the operation, some fans stop rotation, and after continuance of rotation for a certain time for cooling, the drive motor starts to slow down. The drive motor slows down to the number of revolutions preset for the stand-by mode, which revolutions are then continued, that is, the drive motor continues to stand by.
Thus, in the image forming apparatus, if no processing is performed for a while after turning ON of the power, a switching is made into the stand-by mode in several to several ten seconds. But this is for diminishing the power consumption during stand-by. Most drive mechanisms in the apparatus, except fans for heat radiation, come into a rest state. In the stand-by mode, the optical deflector is usually slowed down to the half or so of a predetermined number of revolutions. This is for shortening the time required from when the drive motor starts to rise until when its predetermined number of revolutions is reached, in preparation for the regular operation. According to a certain type of image forming apparatus developed recently, the number of revolutions is decreased to even zero in the stand-by mode for the purpose of further diminishing the power consumption in the same mode.
For performing the image forming processing in the stand-by mode, the operator is required, for example, to depress a button on the control panel to input a processing start signal, whereupon the image forming apparatus goes into an operation mode and the drive motor of the optical reflector starts to rise. The revolution of the motor is increased until reaching the predetermined number of revolutions. At this time, the drive motor of the optical reflector is required to rotate at high speed in a short time from the standpoint of an image forming cycle for example. For this reason, the drive motor of the optical deflector is constituted so as to be used at a higher number of revolutions than that of the usual motors. Its number of revolutions is 5,000 or more, or even 10,000 or more as the case maybe. In this case, a large current flows in the drive motor at the leading edge of the motor to increase the number of revolutions rapidly, with the result that a very loud noise occurs. This noise is a fluctuating noise interlocked with the change in the number of revolutions, which is very offensive to the human ear and causes unpleasant feeling.
After the drive motor has reached the predetermined number of revolutions, the image forming processing is started, and after completion of a series of operations, the various mechanisms of the apparatus come into a rest state. In the event the next processing is not performed even after the lapse of a certain time, a switching is made again into the standby mode and the drive motor slows down. The drive motor eventually stops rotation and assumes a complete standby state.
The noise from the motor is a fluctuating noise interlocked with the number of revolutions, but a human becomes aware of the fluctuation because the human is sensitive to a change of sound. Analysis of a frequency spectrum of the fluctuating noise shows that a gentle distribution is present over a wide frequency band and that sharp peaks projecting from the base spectrum are present in several frequency bands. It is seen that the said sharp peaks fluctuate. Among the sharp peaks, a main-component frequency, which is high in sound pressure level, ranges from several hundred Hz to several kHz. The sound in this frequency band gives rise to a great unpleasant feeling because the human is auditorily sensitive to such sound. That is, this sound is a fluctuating noise of high frequency which causes shrillness. If one hears this fluctuating noise, he recognizes it as a very unpleasant noise because of shrillness.
Heretofore there have been proposed techniques for suppressing this type of unpleasant noises caused by frequency fluctuation, such as, for example, those proposed in Japanese Published Unexamined Patent Application Nos. Sho 63-59797 and Hei 6-175443. According to the "step motor driving method" proposed in Japanese Published Unexamined Patent Application No. Sho 63-59797, the change with time of frequency at the leading edge of a drive motor is like plural curves to mitigate an abrupt change. In the "image forming apparatus" proposed in the Japanese Published Unexamined Patent Application No. Hei 6-175443, at the leading edge of a polygon mirror driving motor, another operation noise is caused to fall down forward to cover the operation noise of the motor, thereby making the motor noise difficult to hear (masking).
According to the above conventional methods, a design is made so that the change of frequency with time describes plural curves in order to eliminate a psychological unpleasant feeling caused by noise at the leading edge of the drive motor in the optical deflector. This construction is somewhat effective in mitigating an abrupt change of sound, but cannot eliminate unpleasant feeling because the frequency fluctuation is almost recognized.
According to the above conventional method in which the motor operation noise is covered with another operation noise to make it difficult to hear, the noise (sound volume) as a whole further increases and causes noisiness. Thus, it is impossible to eliminate unpleasant feeling. Besides, other portions of the image forming apparatus are separately operated continuously during the period from turning ON of the power or termination of the image forming processing until switching into the stand-by mode. This is undesirable from the standpoint of low power consumption.